Annotation RetentionPolicy 区别

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RetentionPolicy做为一个enum类， 有三个值。

1. RetentionPolicy.SOURCE:  其生命周期只存在于source code这个阶段， 在compile的时候， 这类annotation会被JVM所丢弃。当编译完成后， 这类annotation是没有用处的。如： @Override, @SuppressWarnings

   @Target(ElementType.METHOD)
   @Retention(RetentionPolicy.SOURCE)
   public @interface Override {
   }

2. RetentionPolicy.CLASS: RetentionPolicy默认为 CLASS, 当class被加载时， 此类annotation被丢弃。（感觉很少有使用CLASS的情况）
3. RetentionPolicy.RUNTIME： 存在于整个JVM运行环境中。 如：@Deprecated

   @Documented
   @Retention(RetentionPolicy.RUNTIME)
   @Target(value={CONSTRUCTOR, FIELD, LOCAL_VARIABLE, METHOD, PACKAGE, PARAMETER, TYPE})
   public @interface Deprecated {
   }
   

    

Code Example:

importjava.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

/**
 * 
 */
public class RetentionPolicyDemo {

    @Retention(RetentionPolicy.SOURCE)
    @interface RetentionSource {}

    @Retention(RetentionPolicy.CLASS)
    @interface RetentionClass {}

    @Retention(RetentionPolicy.RUNTIME)
    @interface RetentionRuntime {}

    public static void main(String[] args) {
        @RetentionSource
class B {}
        System.out.println(B.class.getAnnotations().length ); // Result: 0
@RetentionClass
class C {}
        System.out.println(C.class.getAnnotations().length ); // Result: 0
@RetentionRuntime
class D {}
        System.out.println(D.class.getAnnotations().length ); // Result: 1
}
}

 只有RetentionPolicy.RUNTIME 返回大于0的length.

 

字节码demo: 

@Retention(RetentionPolicy.CLASS)
@Target(ElementType.TYPE)
public @interface Anno {
}

 

@Anno
public class AnnoTest {
    @Deprecated
    public void test() {
    }
}

 

Constant pool:

   #1 = Methodref          #3.#19         // java/lang/Object."":()V

   #2 = Class              #20            // com/my/java/lang/annotation/retentio

   #3 = Class              #21            // java/lang/Object

   #4 = Utf8              

   #5 = Utf8               ()V

   #6 = Utf8               Code

   #7 = Utf8               LineNumberTable

   #8 = Utf8               LocalVariableTable

   #9 = Utf8               this

  #10 = Utf8               Lcom/my/java/lang/annotation/retentionPolicy/AnnoTest;

  #11 = Utf8               test

  #12 = Utf8               Deprecated

  #13 = Utf8               RuntimeVisibleAnnotations

  #14 = Utf8               Ljava/lang/Deprecated;

  #15 = Utf8               SourceFile

  #16 = Utf8               AnnoTest.java

  #17 = Utf8               RuntimeInvisibleAnnotations

  #18 = Utf8               Lcom/my/java/lang/annotation/retentionPolicy/Anno;

  #19 = NameAndType        #4:#5          // "":()V

  #20 = Utf8               com/my/java/lang/annotation/retentionPolicy/AnnoTest

 

从红色字体中可以看出RetentionPolicy.RUNTIME为RuntimeVisibleAnnotations。  

RetentionPolicy.CLASS为RuntimeInvisibleAnnotations